The factors affecting Hair quality are the Cuticle structure / Moisture Content / Surface Tension / Strength of the hair to name a few.
To know each factor in detail we should know the cause and effect of every action on the hair. In order to help you see effect of our action on the hair there are many instruments available that will help us see these observation to a micro level.
Of course you need to spend sufficient time / energy and money to know these in detail, but for a person who is passionate about quality of hair and who wants to improve on a regular basis, there will be no hurdles for him/her to achieve the goal.
But all said and done at a very professional level there are many other factors which needs to be considered while you are actually buying hair from you hair suppliers. (because you can't carry your lab 24hrs a day)